a) Field of the Invention
The present invention concerns a copolymer of ethylene oxide and at least one substituted oxirane carrying a cross-linkable function, a process for the preparation thereof and the use thereof for producing a solid electrolyte having good mechanical properties, a good cationic conductivity and a good chemical compatibility with the electrodes of a generator which operates with alkali metals such as lithium and sodium.
b) Description of Prior Art
It is known to use solvating polymers for preparing ionically conductive materials. The polymers of ethylene oxide or of dioxolane are polymers which solvate cations, in particular alkali cations such as for example the ion Li+ which is present in rechargeable electrochemical generators of the polymer electrolyte lithium battery type. However, these polymers are semi-crystalline, the rate of crystallinity varying has a function of the molecular weight of the polymer. This semi-crystalline character of the polymers as the consequence of decreasing the conductivity of the materials in which the polymers are present.
It has then been found that it was possible to decrease the crystallinity of semi-crystalline polymers, without affecting their solvating properties and their electrochemical stability, by introducing irregularities in the macromolecular chain at possibly irregular interval. However, it has been observed that the introduction in a semi-crystalline polymer, such as for example a high molecular weight, polyoxyethylene (POE), of units producing irregularities, i.e. replacing of semi-crystalline polymer by a copolymer or a polycondensate, is frequently accompanied by a decrease of the molecular weights and the mechanical properties, for example at high temperature. An attempt was made to overcome this inconvenience by introducing into the polymer, units which enable the formation of tri-dimensional networks by cross-linking the copolymer, before or after its formation. By reason of the restraints imposed by the requirements of electrochemical stability, the particularly preferred units permitting cross-linking are selected among those which contain an unsaturated carbon/carbon bond, such as an allyl bond or a vinyl bond. The introduction of such units into a copolymer, additionally enables to fix various groups, for example ionic groups, on the macromolecular chain.
It is known to prepare copolymers of an ethylene oxide and an oxirane carrying an unsaturated substituent by coordination polymerization by utilizing an initiator based on organometallic derivatives of non alkali and non alkali-earth metals, for example an alkyl aluminum or an alkyl zinc. This type of polymerization is not sensitive to the presence of a small quantity of impurities. However, the reactivity of the different comonomers depends on their steric hindrance. Thus, when there is produced a copolymer of ethylene oxide and an oxirane carrying a saturated substituent (for example propylene oxide) or an oxirane carrying an unsaturated substituent (for example allyl glycidyl ether), the polymerization yield of the ethylene oxide is near 100%, while the yield of the substituted oxirane in a copolymer having a molecular weight higher than 1000 is only 60%. In addition, ethylene oxide is preferably consumed at the start of the polymerization. Because of the difference of reactivity of the monomers, the copolymer formed at the start of the polymerization contains more ethylene oxide and has a higher molecular weight than the one formed in the middle or at the end of the polymerization reaction. The copolymer formed by coordination polymerization thus has long sequences of poly(oxyethylene) which are crystalline and present a high heterogeneity of molecular weights.
It is known to polymerize saturated oxiranes such as ethylene oxide or propylene oxide through an ionic process. When such a polymerization is carried out by means of initiators of the sodium hydroxide or potassium hydroxide type in aqueous solution or in protic solvents such as ethylene glycol, a number of transfer reactions towards the solvent take place, and the molecular weights obtained are very low. When the ionic polymerization of oxiranes is carried out in the presence of initiators of the potassium alcoholate or cesium alcoholate type in an aprotic solvent which solvates cations or in the presence of complexing agents such as crown-ethers, ethylene oxide undergoes a living polymerization, i.e. the average degree of polymerization in number (DPn) increases with the conversion rate, the distribution of the molecular weights is narrow, the polymolecularity index I=Mw/Mn is near 1 and there is practically no transfer and termination reactions. An ionic polymerization which is carried out under these conditions enables to obtain high masses when the polymer is ethylene oxide. However, when used with monomers of the substituted oxirane type, it has only been possible to obtain oligomers to this date. For example, the polymerization of styrene oxide initiated with potassium tert-butanolate gives a poly(oxystyrene) having a mass of 1000 g, and the growth of the poly(oxypropylene) chains is interrupted by reactions of transfer towards the monomer [D-M Simons and U. Verbane, J. Polym. Sc. 1960, 44, 303]. When the monomer is phenyl glycidyl ether, the growth of the chains is also rapidly interrupted by transfer towards the monomer [C. C. Price, Y. Atarachi, R. Yamamoto, J. Poly. Sci. PartA1, 1969, 7, 569). In spite of the advantages associated with the nearly quantitative conversion rates of anionic polymerizations, the prior art shows the living character of the polymerization for ethylene oxide only.
The present invention aims at providing a copolymer of ethylene oxide and at least one substituted oxirane carrying a cross-linkable reactive function, by a free radical process, which enables to obtain an ionically conductive material improved mechanical properties as compared to the materials obtained from known copolymers of the type poly(oxyalkylene), without decreasing the ionic conductivity by an excess number of cross-linking points which would cause an increase of the vitreous transition temperature Tg, said ionically conductive material additionally showing an excellent chemical compatibility with the electrodes of a generator when the material is used as an electrolyte.